This invention relates to a process for producing remarkably high purity nitrogen by adsorption at a low temperature using a starting gas mixture comprising 90% or more of nitrogen and 10% or less of oxygen.
Recent progress in semiconductor industry requires nitrogen having purity of 99.9999% or more in order to avoid undesirable effects of oxygen in the production of high performance semiconductors.
In order to produce high purity nitrogen, there is known a low temperature separation method wherein air is liquefied and purified to yield nitrogen of 99.999% to 99.9999%. But according to this method, the purity of more than 99.9999% can only be obtained by using several tens of rectifying plates. This means that a huge rectifying column is required, which results in increasing in plant cost or becoming uneconomical from the viewpoint of running cost.
Another method for producing high purity nitrogen from air is a pressure swing adsorption method (hereinafter referred to as "PSA method") using a molecular sieving carbon (MSC) for selectively adsorbing oxygen rather than nitrogen. Processes for producing the adsorbent MSC are disclosed in, for example, Japanese Patent Examined Publication Nos. 54-17595 and 49-37036 and Japanese Patent Unexamined Publication No. 59-45914. But according to the PSA method, the purity of nitrogen produced is as low as 99% to 99.9%, so that it is impossible to obtain nitrogen having purity of 99.9999% or more.
On the other hand, the use of A-type zeolite as an adsorbent is disclosed in Japanese Patent Examined Publication No. 55-16088 (U.S. Pat. No. 2,810,454). But according to this reference, argon is purified from a mixture of argon and a very small amount of oxygen by adsorbing the oxygen with A-type zeolite. When nitrogen is included in the mixture of argon and oxygen, the nitrogen is removed by fractional distillation prior to the separation of oxygen. There is no data as to adsorption of an oxygen-nitrogen system nor inventive idea of purifying nitrogen in this reference.